The objective of this proposal is to conduct broad studies of clinical effects of antineoplastic agents, alone or in combination with surgery and/or radiotherapy in children who have malignant diseases. These investigations will be performed according to the dictates of Phase I,II, III and ancillary protocols constructed by committees of the Southwest Oncology Group. Mechanisms of actions and toxicities will be studied as will clinical and laboratory manifestations of drug effect on gross and microscopic disease. The studies will consist of: (1) randomized trials comparing accepted regimens against promising new regimens; and (2) non-randomized trials of promising new regimens for disease conditions no longer responsive to accepted regimens. In both cases, consent will be obtained following explicit explanation.